The present invention relates to polyvinyl chloride resin composition having good adhesion strength to substrates as well as excellent corrosion resistance, stability and discoloring resistance. More particularly, the present invention relates to polyvinyl chloride plastisol composition, in particular polyvinyl chloride resin composition which is suitable for plastisol useful as paints and sealing agents.
Polyvinyl chloride resin composition, which has excellent corrosion resistance and weatherability and exhibits excellent characteristics as a resin for coating, has itself rather poor adhesion strength to various kinds of substrates. For this reason, attempts have been made to improve the Polyvinyl chloride resin composition by adding other resins. As the other resins to be added, there have been known, for example,
(1) polyamide resins,
(2) liquid epoxy resins and polyamide resins,
(3) blocked urethane prepolymer obtained by using a blocking agent composed of urethane prepolymer having isocyanate group from a polyol and an organic diisocyanate, and mono- or polyamide compound having active amino group (Japanese Patent Publication (Kokoku) No. 59-52901), (4) lactam-blocked polyisocyanate and polyamide compound having active amino group (Japanese Patent Publication (Kokai) No. 59-131669), and
(5) a long chain alkyl-phenol-blocked organic diisocyanate polymer (isocyanurate type polyisocyanate) (Japanese Patent Publication (Kokai) No. 62-41278), or oxobenzoic acid ester-blocked organic diisocyanate polymer (isocyanurate type polyisocyanate).
However, addition of these compounds is not sufficient as described below.
That is, addition of the polyamide resins referred to in (1) above is disadvantageous because although adhesion strength increases thereby, there result decrease in the corrosion resistance inclusive of water resistance and solvent resistance due to the presence of amino groups which are active and susceptible to hydration, deterioration of discoloration resistance since the resin composition changes its color into yellow when heated, and decrease in stability.
When the epoxy resin and polyamide resin described in (2) above are added, both of them are highly reactive at room temperature, and they are unstable, and therefore operation tends to be difficult. In addition, discoloration resistance is insufficient although it is improved much more than the case where the compounds in (1) above are added.
When the blocked urethane prepolymer obtained from the polyol and the organic diisocyanate, and the mono- or polyamide compound having active amino group described in (3) above are added, the resin composition is susceptible to the properties of the polyol, one of the essential component of the prepolymer, resulting in that the corrosion resistance including water resistance decreases, and the molecular weight of the resin increases for small amount of effective NCO (NCO%) generated upon heating, which causes problem of instability of the resin composition at high ambient temperatures in summer. Discoloration resistance is also insufficient like the addition of the compounds mentioned of in (2) above.
In the case where the lactam-blocked polyisocyanate and polyamide compound described in (4) above are added, the compatibility is poor because the characteristics of diisocyanate influence strongly on the property of the resin composition, and strong polarity of lactam and presence of large atomic group elevate the dissociation temperature. Therefore, the resin composition suffers poor performance in addition to poor stability due to poor compatibility. Discoloration resistance is also insufficient like the addition of the compounds (2) above.
Although sufficient stability is given by the addition of the long chain alkyl-phenol- or oxobenzoic acid esterblocked diisocyanate polymer described in (5) above to the polyvinyl chloride resin as adhesiveness imparting agent, the dissociation temperature of the resin composition increases due to the presence of phenolic blocking agent and thus it is difficult to bake the resin by heating under the conditions of low temperatures and short period of time (for example, on the order of 120.degree. C. for 30 minutes), and adhesion of the resin is poor. Furthermore, corrosion resistance inclusive of water resistance as well as solvent resistance are poor because no crosslinking agent is used.
Recently, there has been an increasing tendency that coating materials containing polyvinyl chloride sol composition are used in the form of thin films and the thickness thereof become smaller, and accordingly much importance is attached to corrosion resistance and adhesion strength. Discoloration resistance is also considered important. It is therefore very important to balance the above-described three characteristics and stability which are inconsistent to each other. However, conventional techniques are insufficient to cope with the problem.